This invention relates to a method and an apparatus for induction heating an elongated metal member, such as a steel pipe, which is passed through an annular induction heating coil, to obtain a uniform distribution of induction heating temperature over the whole length of the member by means of additional local induction heating against an end portion or portions of the member, whereby uneven heating temperature distribution, which could otherwise take place in the end portion, can be avoided.
Conventionally, continuous induction heating of an elongated metal member, such as a steel pipe, which is being conveyed longitudinally at a predetermined speed, has been performed by passing the metal member 12 through an annular induction heating coil 10, as shown in FIG. 1. The metal member 12 is conveyed at a fixed speed to pass through the coil 12, by means of conveyer rolls 14 which are driven by a motor. The induction heating coil 10 is supplied with AC power of a predetermined frequency from an Ac power source 16 so that a desired or aimed high temperature may be created in the metal member 12. The created temperature can be of the desired or aimed value in the central major portion of the metal member 12 except end portions of some length of the member. In the end portions, however, the temperature differs from that in the major portion because of influences of induction heating property, magnetic field intensity created by the coil 10, inclination in the magnetic filed intensity, frequency of the AC power source, and the shape in the end portions, and thus, in the prior art, uniform heating over the whole length of the metal member 12 including the end portions thereof could not be obtained even with any control in the supply of current. FIG. 2 shows examples of induction heating temperature distribution in the prior art which were taken with respect to a steel pipe having a 610 millimeter outer diameter and a 25.4 millimeter wall thickness. In FIG. 2, curves 18 and 20 show respectively temperature distributions along the length of the pipe which appeared when aimed temperatures were 950.degree. C. and 650.degree. C. As seen from FIG. 2, the value of temperature is the highest at the initial point of the curve which corresponds to the end point of the pipe, then decreases down to a degree below the aimed value, and then increases up to the level of the aimed value, thus making a generally V-shaped curve portion, which is referred to as a so-called V-shape heating phenomenon. The depth of this V shape, i.e., depression from the initial point to the bottom point of V, will be larger as the wall thickness t of a metal member increases, as shown in FIG. 3, and, the width of this V shape, i.e., the distance from the initial point to a point where the other end of the V-shaped curve portion comes to the aimed level, will be larger as the length l of the induction coil increases, as shown in FIG. 4. Such uneven temperature distribution will necessarily have disadvantageous influences on the quality, shape, etc. of the heat treated metal member. In order to avoid such influences on a metal member to be heat treated, in the prior art, metal dummy members of a length have been attached, prior to heat treatment, to both ends of the main metal member to be heat treated so that the V-shape heating temperature region may occur within and be limited to the length of the dummy members without affecting the main metal member. Alternatively, after heat treatment, end portions of a length where V-shape heating took place have been cut away from the main body of metal member. Employment of dummy members, however, requires an enlarged and complicated induction heat treatment system as well as additional steps of attaching and removing the dummy members. The end portion cut-away technique, on the other hand, results in an extreme reduction in production yield as well as the requirement of an additional step of cutting away the end portions.